In a conventional signal processing device in a multiplex communication system, pieces of information are transmitted through a shared transmission path or space in a plurality of channels respectively. As an example of this type of multiplex communication system, communication systems based on frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA) are known.
In particular, in case of the CDMA communication system, a plurality of users can share a frequency band. Therefore, as compared with the FDMA communication system and the TDMA communication system, the number of channels per a certain bandwidth can be increased. Also, because signals of a broad-band frequency are transmitted in the CDMA communication system, adverse influence of frequency selective fading due to multi-path signals is low. Therefore, it is expected that the CDMA communication system is useful for mobile communication.
In a CDMA communication system applied for the mobile communication, two pieces of information different from each other are, for example, transmitted from a mobile station in two channels respectively. For example, Published Unexamined Japanese Patent Application H11-266168 was laid open to public inspection on Sep. 28, 1999. In this patent application, method and device of adjusting an electric power of each transmission signal in the simultaneous transmission of both an audio signal and a data signal is disclosed.
FIG. 1 is a block diagram showing the configuration of a conventional signal processing device (or a base-band modulating device) in the CDMA communication system. In FIG. 1, 31 indicates a spreading modulating unit for performing a direct spreading modulation for two input signals of two channels CH1 and CH2 as a spreading modulation according to two pseudo noise signals input from Code 1 and Code 2. The spreading modulating unit 31 is composed of four exclusive OR gates 31a, 31b, 31c and 31d. 
32 to 35 indicate waveform reshaping units for limiting four modulated signals sent from the spreading modulating unit 31. 36 to 39 indicate channel gain multipliers (hereinafter, called multipliers) for respectively multiplying a reshaped modulated signal sent from one of the waveform reshaping units 32 to 35 by a gain of a gain signal for each channel. 40 indicates an adder for adding together signals sent from the multipliers 36 and 37. 41 indicates an adder for adding together signals sent from the multipliers 38 and 39.
Next, an operation of the conventional signal processing device shown in FIG. 1 will be described below. Two types of signals (for example, an audio signal and a data signal) of two transmission channels CH1 and CH2 are received in the spreading modulating unit 31. The signal of the channel CH1 is received in the exclusive OR gates 31a and 31c, and the signal of the channel CH2 is received in the exclusive OR gates 31b and 31d. 
Also, a pseudo noise signal of the Code 1 is received in the exclusive OR gates 31a and 31d, and a pseudo noise signal of the Code 2 is received in the exclusive OR gates 31b and 31c. 
Therefore, the modulation of quadrature phase shift keying (QPSK) is performed for the two types of signals of the transmission channels CH1 and CH2 in the exclusive OR gates 31a to 31d, and a frequency band of each signal is spread to a spread frequency band which is tens times of the frequency band. In this case, the signals of the channels CH1 and CH2 are modulated in the exclusive OR gates 31a to 31d to produce two modulated signals ICH1 and ICH2 of an I component and two modulated signals QCH1 and QCH2 of a Q component according to following multiplication equations.ICH1=CH1×Code 1ICH2=−CH2×Code 2QCH1=CH1×Code 2QCH2=CH2×Code 1
The modulated signals ICH1, ICH2, QCH1 and QCH2 are orthogonal to each other. The modulated signals ICH1, ICH2, QCH1 and QCH2 spread in the exclusive OR gates 31a to 31d are input to the waveform reshaping units 32 to 35 respectively.
In the waveform reshaping units 32 to 35, waveforms of the modulated signals ICH1, ICH2, QCH1 and QCH2 are reshaped. In detail, an impulse response is superposed on each of the modulated signals ICH1, ICH2, QCH1 and QCH2 to limit the waveband of each modulated signal, and reshaped modulated signals I′CH1, I′CH2, Q′CH1 and Q′CH2 are produced and input to the multipliers 36, 37, 38 and 39.
In each of the multipliers 36, 37, 38 and 39, the reshaped modulated signal I′CH1, I′CH2, Q′CH1 and Q′CH2 is multiplied by a channel gain (or an electric power gain value) βd, βc, βd or βc of a gain signal sent from a signal generating unit (not shown) to produce an electric power controlled modulation signal I′CH1*βd, I′CH2*βc, Q′CH1*βd or Q′CH2* βc.
Thereafter, two types of electric power controlled modulation signals I′CH1*βd and I′CH2*βc are received in the adder 40 and are added together according to a following equation. Therefore, a composite modulation signal Imod is produced. In the same manner, two types of electric power controlled modulation signals Q′CH1*βd and Q′CH2*βc are received in the adder 41 and are added together according to another following equation. Therefore, a composite modulation signal Qmod is produced. That is, the frequency spread modulation signal Imod of the I component and the frequency spread modulation signal Qmod of the Q component are produced.Imode=βd*I′CH1+βc*I′CH2 Qmode=βd*Q′CH1+βc*Q′CH2 
These frequency spread modulation signals Imod and Qmode are converted into analog signals in digital-to-analog converters (not shown) respectively and are input to high frequency modulating units (not shown) respectively. In each high frequency modulating unit, the analog signal is modulated with a high frequency carrier signal to produce a transmission signal, and the transmission signal is output as an electric wave.
In this case, to control an electric power of the transmission signal in the CDMA communication system, the channel gain is sometimes changed during the transmission of the signal.
FIG. 2 shows an example of the change of the channel gain βd of the channel CH1 in the conventional signal processing device shown in FIG. 1. In cases where the channel gain βd is periodically changed to βd1, βd2, βd3, βd4, - - - stepwise at constant time periods without changing the channel gain βc of the channel CH2, an output signal (or an electric power controlled modulation signal) h′ of the multiplier 36 and an output signal (or an electric power controlled modulation signal) k′ of the multiplier 37 have waveforms of h′ and k′ shown in FIG. 2 respectively. Therefore, an output signal (or a composite modulation signal) m′ of the adder 40 has a waveform of m′ shown in FIG. 2, and an electric power value of the transmission signal is changed stepwise.
However, in the conventional signal processing device of the CDMA communication system, the band widths of the signals output from the waveform reshaping units 32 to 35 are limited, and the impulse response is superposed on the modulated signal at each sampling point in the waveform reshaping units 32 to 35. Therefore, there is a transient response in each output signal. Therefore, in cases where the channel gain is changed stepwise, the gain is sometimes changed in the middle of the impulse response. In this case, distortion occurs in the waveform of the transient response, and the bandwidth of the transmission signal is undesirably widened. Therefore, a problem has arisen that the leaking of an electric power of the transmission signal to a signal of an adjacent frequency channel is increased.
The present invention is provided to solve the above-described problem, and the object of the present invention is to provide a signal processing device and a signal processing method of the CDMA communication system in which the leaking of an electric power of a transmission signal to a signal of an adjacent frequency channel is suppressed even though a channel gain for the transmission signal is changed stepwise.